In drilling wells associated with oil and gas exploration, and subsequently when such wells are producing wells, various types of communication techniques and systems have been developed for monitoring and/or controlling well operation. Such systems not only transmit well data related to the operation and functioning of the well, but also transmit operational commands to control valves and other apparatus in the borehole.
One such system, in the form of a wireless, subterranean electromagnetic communication system, is described in Rorden, U.S. Pat. No. 3,967,201, where a relatively low frequency, wireless electromagnetic communication link is established through suitable transmitters and a subterranean antenna.
The subterranean antenna described there included a ferromagnetic core which was surrounded by a solenoidal electrical winding. The core of the antenna was, in fact, formed from a plurality of closely packed 5/8ths inch diameter ferrite rods, each having a circular cross-section and being about ten feet in length. The ferrite rods are described as being potted in a high density polyurethane foam inside of a fiberglass coil, with the solenoidal winding itself being wound around the fiberglass coil and then covered with an insulating material and then with a metal coated mylar shield.
We are also aware of More U.S. Pat. No. 4,216,536, which deals with apparatus for transmitting well data from a downhole location to the surface of the earth and for checking upon the accuracy of that data. This transmission system sends a first set of signals to a computer located at a downhole location. A second set of signals are generated and transmitted to the surface through a second transmission system. When the drilling operation is halted and the drill string brought to the surface, the two sets of data can then be compared. Following such a comparison, the two sets of data are synchronized; and if there is a discrepancy between the data, the discrepancies are analyzed, problems pinpointed, and corrective measures taken to improve the ability of the wireless system to correctly transmit data during subsequent drilling.
Other patents that also generally deal with apparatus for sensing downhole conditions or controlling operations at downhole locations include Connor, Sr., U.S. Pat. No. 3,665,955, Skinner, U.S. Pat. No. 4,426,882, and Maddock, Jr., U.S. Pat. No. 4,483,187.
In dealing with downhole signal transmission, it has become evident that when low frequency signals are being used to provide control over well operations, such as, for example, subsurface safety valves for controlling flow, it is important to provide the precise control signal to accomplish a particular objective at the desired time. It is not desirable to have such a signal generated erroneously as that would not provide the needed control over well operation. Thus, it is important to have a subsurface antenna capable of receiving low frequency electromagnetic signal transmissions from transmitters positioned at surface or other locations, in an accurate manner without interference from downhole conditions as might be created by the material flowing through the borehole. Mechanically induced resonant frequencies in a frequency range within that at which the low frequency electromagnetic signals are to be sent would, of course, produce the possibility of erroneous signal generation. It is desired, therefore, to have a method and structure by which the receiving antenna is mechanically isolated from mechanical motions that might otherwise couple natural resonant frequencies generated thereby into the antenna structure. This is especially desirable where such mechanically generated frequencies would be within the frequency range of the low frequency electromagnetic signals being transmitted to and received by that antenna.
To accomplish the mechanical isolation of the receiving antenna from mechanical motions it was determined to employ a gimballed mounting arrangement for the antenna. Such an arrangement would place the center of gravity of the antenna core very close to or in the plane defined by the balance points of the gimballed mount as is more fully discussed below.
Antenna structures used before the present invention were solid structures similar to that in Rorden where the core and the winding were one piece. It was not possible to effectively isolate such an antenna structure because such a design would be difficult to appropriately balance at its center of gravity due to differential thermal expansion rates of its component parts. That is, one design difficulty was to maintain the balance point over a wide temperature range of thermal expansion effects within the plurality of different materials from which the antenna was constructed, since such differential expansion or contraction would give rise to shifts in the center of gravity. This could occur because the core was formed from a magnetic alloy wound with copper and placed inside a stainless steel tube which was then potted using an epoxy or rubber compound. This variety of different materials all expanding at different rates would cause the center of gravity to shift. There were also problems of how to effectively wire the antenna structure. The wiring to the windings for such an antenna structure would have to be led over any isolating mounting structure and would constitute a spring connection tending to offset the balanced antenna.